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(2020) A Trapdoor in the Carbon Cycle: The Global Implications of Riverine Carbonate Chemistry

Knapp W, Stevenson E & Tipper E


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10f: Room 3, Friday 26th June 05:45 - 05:48

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Submitted by Anne Laura Kruijt on Thursday 25th June 16:36
Dear Will Knapp, can you say anything about the amount of PIC formed in the river that is not 'lost' from the river through deposition, but transported to the river mouth and enters the coastal domain? Is that something your work also looks into? Cheers, Anne A second question I have (sorry I don't see how to upload this as a separate question ) relates to PIC in the coastal domain. Does PIC with a riverine origin have a certain distinct fingerprint (isotopic values of calcium maybe?) that would allow for determining the source of the PIC when found in sediment saples or sediment traps in the coastal ocean/shelf sea (estuarine vs riverine vs further upstream vs coastal, vs brought in from open ocean, etc...)?
Hi Anna, thanks for your questions! To answer the first, if we are able to calculate the amount of Ca lost through deposition, via precipitation and other sinks within the river basin, we will have a pretty good idea of the fraction of Ca remaining in the system, hence the amount reaching the river mouth and ocean. Though this raises an interesting point, that once calcite has been precipitated it is obviously liable to be dissolved back into solution - which is why we need to work on figuring out the residence times of Ca in these catchments and building a reactive transport model when estimating the fCa (fraction of Ca) remaining in the river. To answer your second question; we assume carbonate dissolution is congruent in nature, therefore the waters dissolving the carbonate should only ever inherit the isotopic signature of the lithologies within the river catchment assuming no secondary processes (such as precipitation) are occurring (which they often are). Although a catchment may have a distinct stable Ca isotope signature, it may be hard to isolate it in, for example, a sediment drill core due to processes in the ocean which fractionate these isotopes (i.e. foraminifera/coccolithophores precipitating their tests) masking the riverine isotope signature. It would be neat to be able to do this - but I think it would be harder to constrain the provenance c.f. to more traditional tracers such as radiogenic Sr and Nd isotopes. Hope I managed to answer your questions!

Submitted by Claire Nelson on Friday 26th June 05:08
Hello! Are you planning to measure the 44/40 Ca values of the carbonates with downstream distance to determine how alpha values change, thus further investigating the precipitation rate theory?

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